The present invention relates to a power transmission which comprises a drive-power transmission (for example, a power transmission), which transmits a driving force from an engine to drive wheels, and a frictionally engaging element (for example, a starting clutch) whose capacity for transmitting a driving force frictionally is variably set.
Many automobiles are equipped with such a power transmission, and in the power transmission, a hydraulically actuated frictionally engaging element (for example, a hydraulic clutch) is used for speed ratio control, power transmission control, etc. For example, by controlling the engagement actuation of the hydraulic clutch, the torque transmission can be controlled. For instance, while the vehicle is in a deceleration, the slip factor of the hydraulic clutch is controlled to come into a predetermined range (the slip factor is controlled to allow a little slip), so that the hydraulic clutch can slide to avoid a shock, which may occur from an abrupt change in the torque transmission. Also, the engagement characteristics of the clutch such as a characteristic of engagement initiation are determined and stored in memory in a learning control, which exploits that the torque transmission of the hydraulic clutch calculated from the actuation pressure of the hydraulic clutch equals the actual torque transmission of the hydraulic clutch while the hydraulic clutch is sliding.
Recently, a hybrid-type power transmission, which incorporates an electrical motor generator, has been developed and is coming into practical use. This electrical motor generator, which is connected to the output shaft of the engine, is used to assist the engine in rotational drive as an electrical motor when the vehicle is started. Also, it is used to regenerate energy (by generating electricity) as an electrical generator when the vehicle is in a deceleration. In such a power transmission, if the hydraulic clutch is controlled to slide while the vehicle is in a deceleration, then there is a problem that the efficiency of energy regenerating by the electrical motor generator (energy-regenerating device) is reduced by the amount that corresponds to the sliding of the hydraulic clutch. On the other hand, to keep the efficiency of energy regenerating at a high level, if the hydraulic clutch is controlled to keep a full engagement, then the hydraulic clutch, which does not slide effectively, cannot be used as a device to prevent a shock which may occur from an abrupt change in the torque transmission. Also, the above mentioned learning control, in which the characteristics of the hydraulic clutch are determined and learned for controlling the torque transmission, cannot not be executed as designed.
Conventionally, in the engagement control of the hydraulic clutch, the slip factor of the hydraulic clutch is set at a predetermined value while the vehicle is in a deceleration. With a fixed slip factor, the amount of rotational slide (the difference between the input rotational speed and the output rotational speed) is relatively large while the vehicle is travelling at a high speed with the clutch rotating at a high rotational speed, and conversely, the amount of rotational slide is relatively small while the vehicle is travelling at a low speed with the clutch rotating at a low rotational speed. Here, if the amount of rotational slide is too small for the friction coefficient xcexc to be stable, then the learning control, which involves the determining and learning of the engagement characteristic of the clutch, tends to be inaccurate. For the learning control to be performed accurately, the amount of rotational slide must be maintained within a predetermined range. In a case of prior art where the slip factor is set at a predetermined value for the engagement control of the hydraulic clutch, if the amount of rotational slide is set into a predetermined range appropriate to the learning control while the vehicle is travelling at a low speed, then this setting causes a large amount of rotational slide when the speed of the vehicle increases to a high speed, reducing the efficiency of the energy regenerating. Conversely, if the amount of rotational slide is set into a predetermined range appropriate to the learning control while the vehicle is travelling at a high speed, then only a little amount of rotational slide is available when the speed of the vehicle decreases to a low speed, which condition destabilizes the friction coefficient of the clutch and thereby makes the learning control inaccurate.
To solve the above mentioned problems, it is an object of the present invention to provide a power transmission whose frictionally engaging element provided in a drive-power transmission is adjustable to set the amount of rotational slide to a minimum value within a range that is appropriate to a learning control of engagement characteristic, so that the learning control will be performed accurately without any sacrifice in energy regenerating efficiency.
To achieve this objective, the present invention provides a power transmission that comprises a drive-power transmission (for example, the continuously variable transmission CVT described in the following embodiment), a frictionally engaging element (for example, the starting clutch 5, the forward clutch 25 and the reverse brake 27 described in the following embodiment) and a transmission-capacity controller (for example, the control valve CV described in the following embodiment). The drive-power transmission transmits a rotational driving force from an engine to wheels, and the frictionally engaging element can set variably a transmission capacity for said drive-power transmission. The transmission-capacity controller controls the engagement actuation of the frictionally engaging element. While a vehicle equipped with this power transmission is decelerating, the engagement of the frictionally engaging element is controlled to transmit a rotational driving force from the wheels to the engine. In this instance, the transmission-capacity controller controls the engaging capacity of the frictionally engaging element to bring the difference between the input and output rotational speeds of the frictionally engaging element into a predetermined range. While this difference is within the predetermined range, the transmission-capacity controller stores and renews in memory and learns the controlled amount of the frictionally engaging element as a control value to be used for the next deceleration of the vehicle.
According to the power transmission, which is constructed as described above, while a rotational driving force is transmitted from the wheels to the engine during a deceleration of the vehicle, the transmission-capacity controller controls the engagement of the frictionally engaging element to adjust the difference between the input and output rotational speeds into the predetermined range, where the friction coefficient xcexc of the frictionally engaging element is stable. Because of this adjustment, the engagement characteristic of the frictionally engaging element is determined accurately, which in turn makes the learning control accurate. In this case, because the engagement control of the frictionally engaging element is executed to bring the amount of rotational slide rather than the slip factor thereof into the predetermined range, the engagement characteristic can be determined accurately at any speed of the vehicle. Therefore, the learning control itself can be accurate and reliable.
Furthermore, while a rotational driving force is being transmitted from the wheels to the engine during a deceleration, it is preferable that the transmission-capacity controller control the engagement capacity of the frictionally engaging element to bring the difference between the input and output rotational speeds at first temporarily to a value above the predetermined range and then into the predetermined range. The friction coefficient xcexc of the frictionally engaging element has different values if it is measured by increasing the amount of rotational slide from a complete engagement thereof and if it is measured by decreasing the amount of rotational slide from a slipping condition thereof (because of a phenomenon of hysteresis). To avoid the effect of this hysteresis, in the present invention, after the amount of rotational slide (the difference of the input and output rotational speeds) is made larger, it is made smaller for the learning control. In this way, the engagement characteristic can be determined accurately without any effect of the hysteresis, so the learning control can be also performed accurately.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.